ABSTRACT LINE1 (L1) retrotransposable elements occupy approximately 20% of a mammalian genome. They are mobile DNA elements that can insert into new genomic sites using a copy and paste mechanism. This process, known as retrotransposition, is deleterious at multiple levels such as generation of mutations by inserting into genes, generating DNA breaks and genomic instability, and placing undue burden on transcriptional apparatus. Recent evidence suggests that L1s become activated in aged tissues, certain cancers and in the absence of SIRT6, an aging- relevant sirtuin important for genomic stability and DNA repair. Our preliminary data show that pharmacological inhibition of L1 retrotransposition extends lifespan of a progeroid mouse model. The correlation of L1 activity with aging and age-related phenotypes suggests that L1 activation may contribute to the onset of aging and age-related disease. However, the exact role L1s play in the aging process, what tissues are primarily affected and what aspects of L1 activity contribute to aging remain unclear. As such, the goal of this project is to determine how L1 activity changes in multiple tissues with age and to dissect which aspects of L1 activity are toxic to their host cells. To this end, I will pursue the following specific aims: (1) Determine age- related changes in L1 activity in multiple organ systems by examining DNA, RNA, protein, and translation related activities/products. This aim will develop a comprehensive, organism-wide profile of L1 activity, identifying affected/protected tissues for reference and follow-up studies identifying mechanism of regulation. (2) Determine the consequences of L1 inhibition on cellular and organismal physiology. I will disrupt L1 activity at different stages of L1 lifecycle in primary human cells and in mice. I will use CRISPR technology to reduce the number of genomic L1 copies, shRNA and RNAi to conserved L1 sequences to inhibit l1 expression, and reverse transcriptase inhibitors to inhibit L1 reverse transcription and integration. The proposed work will establish whether L1 activity contributes to age-relative dysfunctions and reduced cellular health. Furthermore, I will determine which aspects of L1 activity are most toxic to the cell, and identify the best mode of L1 inhibition.